High thermal conductivity in silicon nitride with anisotropic microstructure

Silicon nitride was fabricated by tape casting of alpha-Si3N4 powder with 5 wt% Y2O3 and 5 vol% rodlike beta-Si3N4 seed particles, followed by tape stacking, hot pressing under 40 MPa, and annealing at 1850 degrees C for 2-66 h under a nitrogen pressure of 0.9 MPa, Silicon nitrides fabricated by this procedure exhibited a highly anisotropic microstructure with large elongated grains (developed from seed particles) uniaxially oriented parallel to the casting direction, Thermal conductivities parallel to the grain alignment were much higher than those measured in other directions and exhibited high values of up to 120 W/(m . K). The anisotropic thermal conductivity of the specimen could be explained by the rule of mixture, considering that large elongated grains developed from seeds have higher thermal conductivity than a small-grained matrix.